Polyolefins, including polypropylene, are used in a variety of industrial applications. For some of these applications, such as packaging, storage containers, disposable medical devices, and so on, it is necessary or desirable that the product or article be optically clear. The optical and physical properties of the polyolefins are largely dependent upon the number of nucleation sites and the rate of nucleation during the crystallization process. The nucleation step of the overall crystallization process for polyolefins such as stabilized polypropylene is generally slow, so that a relatively small number of nucleation sites are formed. These nucleation sites are the points at which spherulites start to grow during the subsequent crystallization step. Because the number of sites is low, the spherulites can grow to a large size. When the size of the spherulites is larger than the wavelength of incident light, the light scatters and the polyolefin has a hazy appearance.
The size of the spherulites can be reduced and the optical properties of the polyolefin improved by the addition of nucleating and/or clarifying agents ("modifying agents") during processing. These agents increase the number of nucleation sites, and therefore the rate of nucleation. The size of the spherulites that form at these sites is smaller and the spherulites are more numerous as a result of this heterogeneous nucleation. If the spherulites are smaller in size than the wavelength of incident light, scattering of the light is substantially reduced and the resin appears clear.
Reduction of spherulite size also affects the physical properties of the polyolefin, e.g. flexural modulus, heat distortion temperature, impact strength and elongation at break of the resin can change with the addition of a modifying agent. The enhanced heterogeneous nucleation raises the crystallization onset temperature of the resin. This can result in a reduced cycle time during processing and greater manufacturing efficiency.
Nucleating and clarifying agents are generally known in the polymer art. U.S. Pat. No. 5,135,975 describes clarifying agents commercially known as the Millad.TM. group of products produced by Milliken Chemicals. These additives, particularly Millad.TM. 3988 have good organoleptic properties and resist plate-out, but dispersion problems resulting in the appearance of white specks have been encountered by some users. Relatively high process temperatures are needed to prevent the appearance of these white specks. Related clarifiers are described in JP 57-018682 and in JP 86-017834.
U.S. Pat. No. 5,342,868 describes the use of various organophosphorus salts as clarifiers for various crystalline synthetic resins. These additives perform well but their higher cost makes them somewhat less economical for use on a large scale.
There is a need in the art for agents that can be added to polyolefins such as polypropylene that are compatible with the polyolefin and stable under polyolefin processing conditions, that improve the optical and/or physical properties of the polyolefin, and that may be efficiently obtained or produced.